Conventionally, power machineries such as hydraulic shovels, cranes, wheel loaders, and bulldozers (in specification and claims, these power machineries are collectively referred to as “work machines”) have been employed in various fields. For example, a hydraulic shovel will be described. The hydraulic shovel is configured such that an upper rotary body (structure body) is mounted to an upper portion of a lower travelling object, and the upper rotary body includes an engine, a driver seat, an arm attached with a bucket at a tip end thereof, a boom coupled to the arm, etc. Thus, the hydraulic shovel has a great weight. The upper rotary body is rotatable at the upper portion of the lower travelling object by operating a remote control valve provided in the driver seat during a work, and the bucket attached to the tip end of the arm performs various kinds of works.
In recent years, as one example of the work machines, there is proposed a work machine including an electric motor as a drive device for driving the upper rotary body, etc.
For example, there is a work machine in which a hydraulic pump and a generator/electric motor are mounted in parallel to a reduction gear device which is driven by an engine as a driving power source and they are driven, a hydraulic actuator is driven by a hydraulic oil from the hydraulic pump, electric power generated in the generator is stored in a battery, and the generator/electric motor is operated as an electric motor as necessary (e.g., see Patent Literatures 1, 2).
As another prior art, there is a work machine in which a hydraulic pump is attached to an engine as a driving power source, the hydraulic pump is provided with a generator/electric motor via a reduction gear device or a coupling, a hydraulic cylinder or a travelling hydraulic motor is driven by a hydraulic oil from the hydraulic pump driven by a driving power of the engine, and electric power generated in the generator is stored in a battery and used to rotate the rotary electric motor (e.g., see Patent Literatures 2, 3).
As another prior art, there is a work machine in which an engine as a driving power source and a generator are integral with each other, a hydraulic cylinder or the like are driven by a hydraulic oil from a hydraulic pump driven via the generator, and an upper rotary body is driven by the electric motor (e.g., Patent Literature 4).